Early reference to polymerization of vinyl chloride in aqueous emulsion is found in U.S. Pat. No. 2,068,424. Subsequent modifications in technique have included preliminary emulsification of the monomer in a colloid mill with monomer-soluble free-radical donors as initiators, such as described in French Pat. No. 1,065,575. Also, both continuous and discontinuous procedures using water-soluble free-radical donors were exemplarily described in French Pat. No. 1,148,144. However, these methods produce latexes whose mechanical stability is mediocre and to polymers whose properties are not uniform.
Several methods using the technique known as seeding have been developed for controlling the size of the latex particles formed during the course of polymerization. Particle sizes up to about 2 microns are obtainable with a very narrow distribution. Thus, French Pat. No. 989,225 describes the use of seeding to obtain uniform latex particles of diameter larger than 0.5 micron. Seed latexes are prepared by the known techniques of conventional emulsion polymerization. They provide pre-formed particles which serve as nuclei for subsequent polymerization.
In the application of latexes, the constancy of uniformity of product quality is of prime importance. This is particularly so in the case of vinyl chloride polymer latexes which are converted to dispersions in a plasticizer, generally known as plastisols or pastes. The particle size of the resins intended for such applications must conform to very precise criteria. Thus, resins containing considerable amounts of particles whose diameter is less than about 0.1 micron are not appropriate for such applications because they have a tendency to cause very significant increases in plastisol viscosity on standing in storage. On the other hand, if the particles contain a substantial fraction having a size greater than about 2 microns, there is a marked tendency for the plastisol to settle out or to separate into different liquid layers (often termed "syneresis") on prolonged storage awaiting use. Resins whose particle size is between about 0.1 and 2 microns serve particularly well for the preparation of plastisols.
Although a particle size distributed regularly between 0.1 and 2 microns is useful, products of better quality are obtained by mixing populations of different size in certain known proportions. This mixture technique is described in U.S. Pat. No. 2,553,916. By varying the proportion and size of the two populations, it is possible to obtain different desired application properties. Advantageous use can exemplarily be made of mixtures comprising from about 10 to 40% by weight of monodisperse particles having average diameter between about 0.1 and 0.5 micron, with correspondingly about 90 to 60% by weight of monodisperse particles of average diameter between about 0.5 and 2 microns.
Prior techniques directed to the attainment of such mixtures consist in preparing two monodisperse latexes separately; one having particle size between about 0.1 and 0.5 micron and the other having particle size between about 0.5 and 2 microns, the latter being obtained according to the technique known as seeding. The latexes obtained are subsequently mixed in the desired proportions and the mixture submitted to the usual finishing steps such as spray-drying, hot-drum drying, flocculation or others.
However, a particular deficiency of these prior art methods for obtaining monodisperse latexes is that they have not been amenable to working with higher concentrations. In the majority of cases, a dry-solids content of 45% by weight represents the maximum. Furthermore, the tendency of monodisperse latexes to flocculate is a well-recognized fact.
An improvement in procedure, described in British Pat. No. 928,556, consists in mixing two or more seed latexes of different sizes and carrying out the polymerization using the mixture of seed latexes as the nuclei. However, although this technique avoids the final mixing step, it still requires conducting at least three polymerization operations to arrive at the final product, namely preparation of the first seed latex, preparation of the second seed latex and final polymerization using the mixed seed.